The effects of alpha adrenoceptor agonists and antagonists on the postsynaptic alpha receptors were examined in the dog basilar, mesenteric and renal arteries and the type of alpha adrenoceptors present was characterized. In the basilar artery, noradrenaline, clonidine and phenylephrine produced almost the same maximal contraction, the pD2 values being 6.51 +/- 0.11, 5.49 +/- 0.16 and 5.65 +/- 0.13, respectively. Yohimbine (1-3 x 10(-8) M) inhibited the contractile responses to noradrenaline and clonidine competitively and the response to phenylephrine noncompetitively. Corynanthine (10(-6) M) had no effect on such contractile responses. In the mesenteric and renal arteries, the maximal responses to noradrenaline and phenylephrine were markedly greater than those to clonidine. Yohimbine (10(-7) - 10(-5) M) and corynanthine (10(-7) - 10(-5) M) both antagonized noradrenaline competitively in these vessels. In the basilar, mesenteric and renal arteries preloaded with 3H-noradrenaline, 3H-efflux induced by electrical transmural stimulation was attenuated by clonidine (10(-10) - 10(-7) M), while phenylephrine (10(-10) - 10(-8) M) was without effect. Yohimbine at considerably lower concentrations than corynanthine increased the 3H-efflux elicited by electrical stimulation. These results indicate that presynaptic and postsynaptic alpha receptors of the dog basilar artery are largely alpha 2 in contrast to those of peripheral arteries.